Investigations to discover alternative materials for use in catalysis concerning various types of reactions have included evaluation of the suitability of carbide and nitride materials. Generally, carbide and nitride materials have been considered as possible alternatives for use in various types of catalysis since they exhibit “metal-like” properties (e.g., high melting points, hardness and strength). Levy & Boudart report that carbide and nitride materials exhibit catalytic properties similar to those of noble metals. See Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis (Science, 181 (1973), 547-549).
Supported carbide and nitride catalysts have been described generally and reported as suitable for use in various types of reactions. Slaugh et al. describe a supported molybdenum carbide composition prepared by impregnating hexamolybdenum dodecachloride onto a porous aluminous (e.g., Al2O3), siliceous or carbonaceous (e.g., active carbon) support which is then heated in a carbiding atmosphere at a temperature of about 650° C. to about 750° C. See U.S. Pat. No. 4,325,842.
Leclercq et al. report a catalytic reforming process employing catalysts based on tungsten and molybdenum carbides supported on alumina and active carbon. See U.S. Pat. No. 4,522,708. These catalysts are prepared by successive impregnations of active carbon using ammonium molybdate and ammonium tungstate solutions which are evaporated to dryness in air, calcined in a nitrogen atmosphere which is followed by reduction of the tungsten and molybdenum oxides formed during calcination under a hydrogen atmosphere. These compounds are then heated under hydrogen to allow the active phase compounds to react with the carbon support to produce mixed carbides of tungsten and molybdenum.
Sherif et al. report carbon-supported Group VIB metal (e.g., Cr, Mo, W) carbide-containing catalysts formed by calcining a carbon support (e.g., activated carbon and acid washed activated carbon) which has been impregnated with a water-soluble precursor for the metal carbide. See International Publication No. WO 95/32150.
Oyama reports interstitial alloys formed by the incorporation of carbon, nitrogen, and oxygen into the lattices of early transition metals to produce a class of compounds with metallic character. See Preparation and Catalytic Properties of Transition Metal Carbides and Nitrides (Catalysis Today, 15, 179-200. 1992).
Iwai et al. report carbonitrides consisting of a carbide and nitride of the metals of Groups IV, V, and VI prepared by calcining a precursor obtained by reacting polyphenol with the reaction product of ammonia and the halide of a Group IV, V, or VI metal. The precursor may also be obtained by reacting the reaction product of polyphenol and the halide of a Group IV, V, or VI metal with ammonia. See U.S. Pat. No. 4,333,916.
Faubert et al. report on methods for preparing iron-containing catalysts containing iron carbide particles prepared by activation of a precursor consisting of Fe hydroxide adsorbed on carbon black by hydrogen reduction and pyrolysis in the presence of acetonitrile. See Activation and characterization of Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells (Electrochimica Acta, Vol. 43, Nos. 14-15, pp. 1969-1984, 1998).
Cote et al. report on methods for preparation of non-noble metal based catalysts prepared by pyrolysis of a transition metal hydroxide (e.g., vanadium, chromium, iron, cobalt hydroxide) on carbon black including reduction in the presence of hydrogen and heating in the presence of acetonitrile. See Non-noble metal-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells (Journal of New Materials for Electrochemical Systems, 1, 7-16, 1998).
Catalysts containing carbides or nitrides may be advantageous in certain instances due to the absence of a costly noble metal. One such reaction in which an active catalyst which does not require the presence of a noble metal may be advantageous is the oxidation of a tertiary amine (e.g., N-(phosphonomethyl)iminodiacetic acid) to produce a secondary amine (e.g., N-(phosphonomethyl)glycine). N-(phosphonomethyl)glycine (known in the agricultural chemical industry as “glyphosate”) is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in an aqueous formulation. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz (U.S. Pat. No. 3,950,402) teaches that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as “PMIDA”) with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support:

Other by-products also may form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product; and aminomethylphosphonic acid (“AMPA”), which is formed by the oxidation of N-(phosphonomethyl)glycine. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., “leaching”) result because under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form and both PMIDA and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal.
In U.S. Pat. No. 3,969,398, Hershman teaches that activated carbon alone, without the presence of a noble metal, may be used to effect the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine. In U.S. Pat. Nos. 4,624,937, Chou further teaches that the activity of the carbon catalyst taught by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formaldehyde by-product when used to effect the oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid. This formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as “NMG”) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
It has been suggested that the formaldehyde be simultaneously oxidized to carbon dioxide and water as the PMIDA is oxidized to N-(phosphonomethyl)glycine in a single reactor, thus giving the following reaction:

As the above teachings suggest, such a process requires the presence of both carbon (which primarily effects the oxidation of PMIDA to form N-(phosphonomethyl)glycine and formaldehyde) and a noble metal (which primarily effects the oxidation of formaldehyde to formic acid, carbon dioxide and water). Previous attempts to develop a stable catalyst for such an oxidation process, however, have not been entirely satisfactory.
Like Franz, Ramon et al. (U.S. Pat. No. 5,179,228) teach using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), however, Ramon et al. teach flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%. Still, the amount of noble metal loss incurred with this method is unacceptable. In addition, re-depositing the noble metal can lead to loss of noble metal surface area which, in turn, decreases the activity of the catalyst.
Using a different approach, Felthouse (U.S. Pat. No. 4,582,650) teaches using two catalysts: (i) an activated carbon to effect the oxidation of PMIDA into N-(phosphonomethyl) glycine, and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde into carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl) glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl) glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of PMIDA to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle.
Ebner et al., in U.S. Pat. No. 6,417,133, describe a deeply reduced noble metal on carbon catalyst which is characterized by a CO desorption of less than 1.2 mmole/g, preferably less than 0.5 mmole/g, when a dry sample of the catalyst, after being heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20° to about 900° C. at a rate of about 10° C. per minute, and then at about 900° C. for about 30 minutes. The catalyst is additionally or alternatively characterized as having a ratio of carbon atoms to oxygen atoms of at least about 20:1, preferably at least about 30:1, at the surface as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500° C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
The catalysts of U.S. Pat. No. 6,417,133 have proven to be highly advantageous and effective catalysts for the oxidation of N-(phosphonomethyl) iminodiacetic acid to N-(phosphonomethyl) glycine, and for the further oxidation of by-product formaldehyde and formic acid, and without excessive leaching of noble metal from the carbon support. It has further been discovered that these catalysts are effective in the operation of a continuous process for the production of N-(phosphonomethyl)glycine by oxidation of N-(phosphonomethyl) iminodiacetic acid.
Carbon and noble metal sites on the catalysts of U.S. Pat. No. 6,417,133 are highly effective for transfer of electrons in the oxidation of N-(phosphonomethyl)iminodiacetic acid, and the noble metal sites are especially effective for this purpose in the oxidation of formaldehyde and formic acid. However, it would be advantageous to have a multi-reaction catalyst and reaction process which oxidizes PMIDA to N-(phosphonomethyl)glycine while simultaneously exhibiting desired oxidation of formaldehyde to carbon dioxide and water (i.e., increased formaldehyde activity), and which does not require the presence of a noble metal (e.g., a carbide, nitride, or carbide-nitride containing catalyst). Additionally or alternatively, it would likewise be advantageous to have such a multi-reaction catalyst and reaction process which does not require costly noble metal, or which functions effectively with a reduced noble metal content relative to catalysts currently available for commercial manufacture of N-(phosphonomethyl)glycine or other secondary amines.
Salts of iminodiacetic acid may be phosphonomethylated to form PMIDA which, in turn, may be oxidized to form N-(phosphonomethyl)glycine in accordance with the above description. See, e.g., Gentilcore, U.S. Pat. No. 4,775,498 (disclosing a method to phosphonomethylate a salt of iminodiacetic acid); Ebner, et al., U.S. Pat. No. 6,417,133 (disclosing methods for oxidizing PMIDA).
Salts of nitrilotriacetic acid, for example, are excellent chelating agents, and consequently may be used as detergent builders, water-softening agents, scouring aids, dyeing assistants, paper-coating agents, scale inhibitors, and agents for preventing soap degeneration. And many amino-carboxylic acid salts (e.g., salts of glycine, salts of iminodiacetic acid, etc.) may also be neutralized to their corresponding acids and then used, for example, as chelating agents; in food preparations; and as raw materials for making pharmaceuticals, agricultural chemicals, and pesticides. See, e.g., Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 234-41 (disclosing the use of glycine and iminodiacetic acid compounds as raw materials to form N-(phosphonomethyl)glycine).